The bypass flap is a key element in the exhaust gas recirculation (EGR) system.
Its function is to direct the EGR gases into a bypass circuit of the exchanger in order to exploit hot gases in priming the catalyst.
The correct operation of the flap therefore makes it possible to guarantee the depollution of current diesel engines. Blocking of the flap in bypass mode or in cooled mode has direct consequences on the pollution emitted from the engine outlet.
Since pollution thresholds are becoming increasingly strict, it is vitally important, in order to satisfy the next standards, to diagnose such failures of the flap.
The risk associated with the blocking of the flap is also not linked solely to pollution. In practice, a failure of the flap can have consequences on the reliability of the surrounding components (degradation due to an excessively high temperature of the EGR valve and its mounting) and the integrity of the engine control strategies that use it (such as, for example, clearing the valve and the exchanger, or even priming the catalyst).
A number of failure diagnosis methods have already been developed, with variable performance levels.
A first method, disclosed in the document JP2006-291921, uses a temperature sensor situated at the inlet of the intake distributor and makes it possible to diagnose a blocking of the flap by measuring the temperature difference between the cooled mode and the bypass mode. However, this method based on a simple difference between two temperatures, offers little robustness with respect to the dispersions and variations associated with the inertia of the EGR circuit. Furthermore, this method has the drawback of not making it possible to check the correct closure of the flap after actuation: a blocking of the flap in the actuated position will therefore be detected only on the next occurrence of the diagnosis. Moreover, this method appears relatively imprecise because the temperature sensor situated at the inlet of the intake distributor is subject to the influence of the cool air intake.
Another method, described in the document JP 2003-247459, implements a strategy based on monitoring the air flow rate before and after activation of the bypass flap, the air intake flap and the EGR valve being totally open. The main advantage of this solution is that it simply uses the flow meter situated on the cool air intake duct. However, depending on the technical means employed, this strategy may result in a not-inconsiderable false detection ratio, due to the EGR environment (high temperatures, fouling of the connections) and to the limited reactivity of the flap control. In practice, pressure wave phenomena delay the control through the vacuum of the bypass flap.
Finally, other applications use a contactor that makes it possible to know the open/closed position of the flap. However, this strategy can also lead to a high false detection ratio, because of the EGR environment.
One aim of the invention is therefore to define a simple and reliable method that makes it possible to detect any failure of the bypass flap. Another aim of the invention is to diagnose a total loss of the cooling function.